Spacecraft, such as satellites and space probes, are steered by so-called "thruster" engines that are ignited and burned for carefully controlled intervals of time. Separate thruster engines are arranged with respect to the center of mass of the spacecraft to control movement of the craft in three dimensions.
Fuel delivery to the thruster engines is controlled by respective solenoid valves that are normally biased to closed positions for preventing any fuel from reaching the engines. However, when steering is required, one or more of the solenoid valves is energized to an open position for permitting the delivery of fuel to selected thruster engines. The solenoid valves remain energized for limited intervals of time through which the engines effect the desired maneuver.
The thruster engine valves are preferably made as lightweight as possible to help minimize launch weight of the spacecraft. However, redundant parts are sometimes used to assure that the valves close properly to cut off the supply of fuel to the engines. Although the redundant parts add weight to the valve, such redundancy is important because a single thruster engine that fails to shut off can send a satellite into an uncontrolled tumble or into a trajectory that could destroy the craft.
Another requirement that adds weight to the valves is related to the rigors of space flight. Movement of the spacecraft, especially during launch and atmospheric travel, subjects the thruster engine valves to severe vibration and shock. Heavy springs are required to withstand the vibration and shock on the relatively moving parts of the valves for maintaining the valves in a closed position. However, the same spring forces must be overcome by solenoids to open the valve. Accordingly, other weight is added to the valves by increasing the size of the solenoids to overcome the spring forces.
In addition to the special requirements for using the thruster engine valves in space, the valves must also withstand especially aggressive testing procedures before they are used in spacecraft. The valves are cycled, i.e., opened and closed, several thousand times for testing the valves and the engine systems with which they are used. Often the tests are conducted without any fuel passing through the valve. These so-called "dry cycling" procedures subject relatively moving parts of the valves to severe abrasion.
Most of the wear occurs between the relatively moving parts of the solenoid, namely, the solenoid plunger and the bore that guides the plunger. In addition to attracting the plunger toward a pole piece at one end of the bore, magnetic forces of the solenoid also attract the plunger toward the sides of the bore. Over time, repeated energization of the solenoid (i.e., cycling) causes galling between the plunger and bore. Particles worn away by the galling contaminate both the valve and the fuel supply. Within the valve, the particles can accelerate the wear between the plunger and bore or become lodged in positions that prevent the valve from sealing properly to cut off the fuel supply.
Wear between the plunger and bore can also be aggravated by clearance between the plunger and bore. The plunger and bore are relatively dimensioned to provide sufficient clearance between them to allow the plunger to move along the bore. However, it is also desirable to minimize the clearance to prevent the plunger from cocking within the bore, because such cocking can increase wear at the ends of the plunger and more rapidly wear the bore. Cocking can also prevent the valve from closing properly. Although narrow tolerances are desirable to minimize cocking between the plunger and bore, finishing operations, such as honing, that are required to achieve the narrow tolerances within the bore are generally not permissible because of the contamination that they cause. Small particles of the finishing grit and bore stock can become embedded in portions of the bore and resist attempts to clean them out. However, the same particles can dislodge during use of the valve and cause the same kinds of contamination problems caused by the wear particles.
Accordingly, the bores of the thruster valves are usually made to a wider tolerance than desired by using boring operations that produce larger chips that are more easily cleaned out of the bore. However, even the wider tolerances are difficult to achieve because the bore is made of dissimilar materials (i.e., magnetic and nonmagnetic) along its length having different machining characteristics.
Another way of limiting cocking between the plunger and bore is to increase the length of the plunger with respect to the bore diameter to an aspect ratio of at least 1 to 1. The longer length of the plunger decreases the angular amount that the plunger can cock through a given clearance between the plunger and bore. However, the additional length of the plunger adds weight to the valve. Further, the additional mass of the plunger requires a heavier spring to maintain the plunger in a biased position along the bore, and the heavier spring requires a larger solenoid to overcome the spring force and open the valve.